The polar modulator is a good architecture for a radio transmitter when moving to more digitally intensive implementations. A transmitter for the Global System for Mobile Communications (GSM) and for Enhanced Data rates for GSM Evolution (EDGE), and comprising a polar modulator, is disclosed in “Spur-free all-digital PLL in 65 nm for Mobile Phones”, B. Staszewski et al, IEEE International Solid State Circuits Conference, session 3.1, 2011. A transmitter for Wideband Code Division Multiple Access (WCDMA), and comprising a polar modulator, is disclosed in “A fully digital multimode polar transmitter employing 17b RF DAC in 3G mode”, Z. Boos et al, IEEE International Solid State Circuits Conference, session 21.7, 2011. Such polar transmitters can be very power efficient. However, they are less well suited to use in wider band radio systems such as the Third Generation Partnership Project Long Term Evolution, referred to, for brevity, as LTE.
A problem with polar transmitters is that the bandwidth of the phase and amplitude modulation signals used for modulating the transmitter is much wider than the bandwidths of in-phase and quadrature-phase (I/Q) components of a modulation signal. Typically the bandwidth required to achieve good Adjacent Channel Leakage Ratio (ACLR) and Error Vector Magnitude (EVM) performance in a polar transmitter is three to four times higher than in a transmitter implementing modulation by I/Q components.
An additional problem with a polar transmitter is that the bandwidth of the composite signal, that is, a carrier signal after modulation by both the amplitude and phase modulation signals, is wider than the bandwidth of both the amplitude and phase modulation signals. This is because the amplitude and phase modulation signals are multiplied during modulation of the carrier signal, which is equivalent to convolution in the frequency domain.
This is especially a problem with systems such as LTE, which have a high modulation bandwidth. It is important in frequency division duplex (FDD) systems, such as LTE, as well as WCDMA, that the transmitter of a wireless communication device does not desensitise the receiver of the same device. A low ratio of duplex distance to modulation bandwidth can present a challenge. In WCDMA, the worst ratio of duplex distance to modulation bandwidth occurs when the duplex distance is 45 MHz and the modulation bandwidth is 3.84 MHz. In LTE, the worst ratio of duplex distance to modulation bandwidth occurs when the duplex distance is 80 MHz and the modulation bandwidth is 18 MHz. Therefore, in LTE the worst ratio of duplex distance to modulation bandwidth is 2.6 times smaller than in WCDMA, making it very challenging to build a polar modulator.
Another challenge when building a polar modulator for LTE is the very high frequency deviation required in a 2-point phase locked loop (PLL). In WCDMA, the required deviation is about ±10 MHz, and in LTE it is ±50 MHz. It is very challenging to generate such a wide linear tuning range and also a wide deviation reduces the available tuning range, limiting the ability to support multiple bands in one digitally controlled oscillator (DCO).
Yet another challenge with polar modulation is the time alignment requirement. When combining the amplitude and phase modulation signals, the timing must be very accurate, otherwise spectral growth and violation of transmitter ACLR and the receiver band noise requirements will occur.